In the radial turbine which is typically employed as an exhaust gas turbine of a turbosupercharger, it is often desirable to be able to supercharge the engine even at a relatively low engine speed and this can be achieved by increasing the velocity of the exhaust gas entering the turbine by restricting the cross section of the passage leading to the turbine wheel. However, when the passage is restricted, the inlet pressure of the turbine or the back pressure of the engine exhaust system rises and the efficiency of the engine drops.
Therefore, if a plurality of moveable vanes are arranged in a throat located adjacent the outer circumference of a turbine wheel in a circle so that the area of the nozzles defined between the moveable vanes may be varied by rotating the moveable vanes over a certain angle as described in Japanese Patent Publication No. 38-7653, it is possible to ensure supercharging of the engine in a low speed range of the engine and, at the same time, to keep the back pressure of the engine exhaust system to a low level in a medium to high speed range of the engine.
The Japanese Patent Publication discloses a mechanism for allowing the rotational motion of moveable vanes comprising an arm member fixedly attached to each of the nozzle vanes which are constructed as moveable vanes and an annular drive member which is disposed concentric to the turbine wheel and is engaged to the arm members in such a manner that by causing rotational motion to the annular drive member the moveable vanes can be rotated in mutual synchronization.
According to this structure, since the annular drive member has to be placed around the main shaft of the turbine wheel, the drive member tends to interfere with the casing for a lubrication unit and other parts of the turbine and the overall size of the turbosupercharger can not be reduced as much as desired. Further, for accurate synchronization of the vanes, the component parts need to be manufactured at high precision and the overall manufacturing cost tends to rise.
Furthermore, due to the complexity of the mechanism for controlling the motion of the moveable vanes, accurate positioning of the moveable vanes in particular when the nozzle opening defined by the moveable vanes are small and the width of the nozzle openings is very critical to the performance of the turbine is difficult. Conventionally, the most closed position of the moveable vanes is determined by a stopper means provided in the annular drive member as disclosed in Japanese Patent Laid Open Publication No. 50-94317 or, alternatively, by shoulders which engage the axial edges of the moveable vanes as disclosed in Japanese Patent Laid Open Publication No. 50-94317. According to these conventional ways of defining the most closed positions of moveable vanes, it is difficult to accomplish smooth synchronization of the moveable vanes and accurate positioning of the moveable vanes when they are at their most closed positions.
Further, in order to allow some tolerance to the component parts it is desirable that the most closed positions of the moveable vanes can be adjusted, but the conventional structure for actuating the moveable vanes were not suitable suitable for such an arrangement.